1. Field of the Invention
The present invention relates to a fine-particle classification apparatus for classifying fine particles in an aerosol in a gas phase, and more particularly, to a fine-particle classification apparatus for charging target fine particles in the aerosol and further applying an electrostatic field to the particles, thereby classifying the particles using a difference between each mobility which depends on the particle diameter.
2. Description of the Related Art
As a system for fine-particle classification apparatus for classifying fine particles in an aerosol, there have been a variety of types conventionally. The following describes about the Differential Mobility Analyzing (hereinafter referred to as DMA) which is the first conventional example for the fine-particle classification apparatus. In DMA, when fine particles in an aerosol are classified, target particles are first charged. The charged target fine particles are next applied an electrostatic field. The classification is performed using that the mobility of a particle in a medium gas is different depending on the particle size (diameter). The details for DMA are described, for example, in Journal of Aerosol Science, Vol.28, No.2, pp.193 to 206, 1997.
The following next describes the Particle Beam Mass Spectrometry (hereinafter referred to as PBMS) which is the second conventional example for the fine particle classification apparatus. In PBMS, target particles are focused into a beam with an ultra-sonic velocity in a process where enclosed fine particles are injected from a particle source to an ultra-high vacuum environment. The beam with the ultra-sonic velocity of fine particles is next charged in an electron beam. The charged target fine particles are then applied an electric field in the ultra-high vacuum environment, thereby performing a classification corresponding to a mass of a fine particle. The details of PBMS are described, for example, in Journal of Aerosol Science, Vol.26. No.5, pp.745 to 756, 1995.
In the first conventional example, it is necessary that an operation gas pressure in the DMA type classification apparatus be high, for which one of the reason is that the development thereof was started in the premise that an aerosol with an atmospheric pressure was sampled to classify. Therefore, it is considered that the operation gas pressure with more than a range of 50 to 100 Torr be necessary even in a recent reduced pressure DMA type classification apparatus.
The classification accuracy for the DMA type classification apparatus is determined by the degree of Brownian diffusion of the target fine particle in the aerosol. In detail, a great degree of Brownian diffusion means a great displacement by a fluctuation of the fine particle. When the Brownian diffusion of the fine particle is great, it is not possible to perform an accurate classification. Accordingly, when an inert gas with a low gas pressure and a small mass as a medium gas in the DMA type classification apparatus, the degree of Brownian diffusion becomes great, and the degree of classification accuracy in the DMA type classification apparatus deteriorates. Therefore, it is desired to use an inert gas with a high gas pressure and a great mass to some degree as the medium gas inside the DMA type classification apparatus.
On the other hand, it is necessary to lower a gas pressure inside an aerosol generation apparatus for generating an aerosol containing fine particles at a gas phase. In order to generate nm-sized fine particles, in particular, with a particle diameter of less than 10 nm to produce functional materials, it is desired to prepare inert background gases with a small mass as possible, and to make the gas pressure less than 50 Torr, inside the aerosol generation apparatus. It is because when inert background gases with a high gas pressure and a great mass are prepared inside the aerosol generation apparatus, generated fine particles are aggregated and grown to large sizes thereof.
The thus generated fine particles are classified, and the classified fine particles are deposited on a substrate, thereby producing the functional materials. In this case, since the process for depositing the fine particles on the substrate is performed after the classification process, it is necessary to flow the fine particles in the DMA type classification apparatus from the aerosol generating apparatus. It is effective to use a differential pressure introduction to flow the fine particles in the DMA type classification apparatus from the aerosol generation apparatus. Therefore, it is necessary to lower a pressure inside the DMA type classification apparatus than that inside the aerosol generation apparatus.
However, as described above, it is desired that the gas pressure inside the DMA type classification apparatus be high to improve the classification accuracy. There is thus a problem that it is difficult to improve the classification accuracy in the method of introducing the aerosol from the aerosol generation apparatus to the DMA classification apparatus using the pressure difference.
On the contrary, in the second conventional example, since the inside of the PBMS type classification apparatus is set to a high vacuum, the pressure inside the PBMS type classification apparatus is lower than that inside the aerosol generation apparatus.
However, in order to classify the fine particles with a diameter of several nm without lowering the yield in the PBMS type classification apparatus, aerodynamic lenses are needed to focus fine particles into a beam in the process that the fine particles are injected from a source. It is very difficult to design the aerodynamic lenses with a high yield and a low dispersion of the kinetic energy. Further, it is necessary that the electric grounding of walls of a vacuum chamber in the PBMS type classification apparatus be set at a level equal to or less than 0.1 volt (V) over the entire apparatus in order to keep a size of the PBMS type classification apparatus within a size range for practical use. There is thus another problem that the production of the PBMS type classification apparatus is very difficult.
An object of the present invention is to provide a fine-particle classification apparatus capable of introducing an aerosol inside the fine-particle classification apparatus with a total pressure equal to or higher than that inside the aerosol generation apparatus, and classifying fine particles from the introduced aerosol.
The main subject of the present invention is to increase a carrier gas velocity in a take-in section for introducing the aerosol to the fine-particle classification apparatus from the aerosol generation apparatus so as to decrease a static pressure in the take-in section, thereby introducing the aerosol inside the fine-particle classification apparatus with a total pressure equal to or higher than that in the aerosol generation apparatus from a fine particle generating area, i.e., aerosol generation apparatus with the total pressure equal to or lower than that in the fine-particle classification apparatus.
Further, it is preferable in the present invention that the take-in section made at a side of the fine-particle classification apparatus have a piping structure in which a specific carrier gas flows, and that a diameter of the introduction section be smaller than diameters of pipes connected to the front and back portion of the take-in section, whereby the carrier gas velocity is increased locally in the introduction section. As a result, it is possible to lower the static pressure in the introduction section effectively.
It is further desired in the present invention to introduce, as a carrier gas or sheath gas inside the fine-particle classification apparatus, a medium gas of which the kind is different from that of the medium gas used in the aerosol generation apparatus, in particular, the medium gas with a mass greater than that of the medium gas used in the aerosol generation apparatus.
It is thus possible to lower the static pressure in the introduction section further effectively. As a result, it is possible to introduce the aerosol to the fine particle classification apparatus further efficiently. Further, by the use of the medium gas with a great mass as the medium gas (carrier gas or sheath gas) inside the fine-particle classification apparatus, it is possible to suppress Brownian diffusion of target particles in the fine-particle classification apparatus, thereby making it possible to improve the classification accuracy in the fine-particle classification apparatus.